None.
1. Field of the Invention
The present invention relates to marine cables with sensor units for detection of reflected acoustical energy.
2. Description of the Related Art
Conventional seismic surveys are conducted by a vessel towing an acoustic source close to a water surface. This source emits acoustical energy, typically in the form of pressure waves (p-waves), that penetrates a geological formation of interest below a seabed or mud line (xe2x80x9csubsurface formationxe2x80x9d). Some of the emitted energy is reflected when the pressure waves pass through the boundaries between the different layers in the ground. A network of sensors (e.g., hydrophones) mounted in one or more cables registers reflected acoustical energy in the form of p-waves. These cables are typically towed in conjunction with or separately from the acoustic source. Conventionally, the hydrophones are pressure sensitive electrical sensors that generate electrical signals corresponding to the detected acoustical energy.
Parts of the energy in the emitted p-waves are reflected as shear waves (s-waves). By registration of the s-waves one can get additional information about the subsurface formation. S-waves propagate only through solids, not liquids. Therefore, the sensors configured to detect s-waves are ideally placed at the seabed to record this type of signals. This is known technology, and the method is amongst other described in U.S. Pat. No. 4,725,990. The sensors that are used have to be motion sensitive (e.g., geophones or accelerometers).
The sensor units"" acoustic coupling to the seabed is a critical factor for measuring s-waves. Most known solutions are based on the sensor units"" gravity to acoustically couple the sensor unit to the ground. By acoustically couple, it is meant that s-waves (or other forms of acoustically energy) are transferred to the sensor unit from an adjacent body (e.g., seabed). Several solutions have been proposed to optimize the acoustic coupling, amongst other U.S. Pat. No. 5,275,066 where it is claimed that the sensor unit has to be heavy. However, it is believed that a heavy sensor unit will have limitations in the frequency band that can be recorded. A heavy sensor typically acts as a filter for the high frequencies because the shear forces in the seabed will not be large enough to move the sensor at high frequencies. Patent No. 307482 (Norway and WO 00/29874) describes a solution where the sensor unit has to be substantially heavier per unit of length than the cable between them. With relatively thick and heavy cables this will result in a heavy sensor, and it may not be able to record the highest frequencies. According to known theory, the sensor unit should have a specific gravity equal to the specific gravity of the seabed to achieve optimum measurements. Moreover, tests have shown that a sensor with higher specific gravity than the seabed give acceptable results as long as the sensor does not have very high specific gravity. To the inventors"" reading, Patent No. 307482 does not appear to explain why the ratio between the weight of the cable and the sensor has to be as described. It is presumed that the intention is to make a system where the vibration in the cable is not transferred to the sensor unit. Other relevant discussions are found in GB 1,385971, 2,202,956 and 2,247,527, U.S. Pat. No. 5,575,187 and WO 97/19846.
To make most advantageous use of reflected shear waves, there is a need for a simple sensor unit that can detect shear waves in an extended range of frequencies.
The present invention provides devices, systems and methods for enhanced detection of reflected acoustical energy, particularly those in the form of shear waves in a seabed. A preferred system for marine seismic exploration includes a first vessel for deploying a seismic source in a body of water, and a second vessel for deploying a cable adapted to rest on a seabed. One or more sensor units disposed along the cable are configured to detect reflections of the acoustical energy emitted by the source. A preferred sensor unit includes a sensor for detecting shear waves (e.g., a multi-axis accelerometer) and a pressure wave sensor (e.g., hydrophone). The sensor unit includes electronics circuitry for processing and transmitting the signal output of the sensors. Preferably, the cable includes a signal/data transmission carrier to allow the exchange of data between the sensor units and surface equipment. The signal transmission carrier, which can be formed of wires configured to transfer electrical signals, optical signals, and/or power, is preferably coupled to the sensor unit with a subsea connector.
Each sensor is disposed in a decoupling device that substantially acoustically uncoupling the sensor units from the cable. One preferred decoupling device for providing isolation or uncoupling from reflecting acoustical energy in the cable (e.g., shear waves) includes constituent structures that are substantially less rigid than the cable. In one embodiment, the decoupling device includes one or more one flexible tension member having opposing ends connected to the cable. The sensor unit is affixed to the flexible tension member with a fastening member. Optionally, the fastening member is at least partially formed of a vibration absorbing material. Additionally, a spacer can be affixed to the tension member to selectively adjust the resonant frequency of the tension member. Further, a resilient tube can be used to partially or fully enclose the sensor unit and associated equipment.
In another aspect, the present invention provides a method for performing marine seismic surveys. In one preferred embodiment, the method includes disposing a sensor unit in a cable, positioning the sensor unit on a seabed such that the sensor unit is acoustically coupled to the seabed, and connecting the sensor unit to the cable with a decoupling device that substantially acoustically uncouples the sensor unit from the cable. The sensor unit can be configured to detect reflected acoustical energy that are in the form of shear waves and/or pressure waves. Preferably, the method includes substantially isolating the sensor unit from movement in the cable that is associated with acoustical energy. In other embodiments, the method includes transmitting data signals indicative of the detected reflected acoustical energy via at least one data transmission carrier to a surface vessel.
It should be understood that examples of the more important features of the invention have been summarized rather broadly in order that detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.